Electromagnetic thermal current circuit breaker



Nov. 12, 1963 3,110,786

ELECTROMAGNETIC THERMAL CURRENT CIRCUIT BREAKER F. L. GELZHEISER Filed Nov. 5, 1959 5 Sheets-Sheet 1 Nov. 12, 1963 F. GELZHEISER 3,110,786

ELE TROMAGNETIC THERMAL CURRENT CIRCUIT BREAKER Filed Nov. 3, 1959 3 Sheets-Sheet 2 9 j 39 35 l9 '7 INVENTOR 37 3| Fig.4. Francis L. Gelzheiser ATTORNEY Nov. 1.2, 1963 F. GELZHEISER' 3,110,786

ELECTROMAGNETIC THERMAL CURRENT cmqun BREAKER 3 Sheets-Sheet 3 Filed NOV. 3, 1959 Fig.|O.

United States Patent 3,11% 756 ELECTRGMAGNETHC TEERlvTAL QURRENT This invention relates to automatic electric circuit breakers and more particularly to current responsive tripping means for effecting the automatic opening of such circuit breakers upon current overload.

An object of this invention is to provide a circuit breaker embodying a thermal magnetic trip mechanism that trips magnetically upon the occurrence of relatively low overload currents.

Another obje t is to provide a circuit breaker embodying a thermal magnetic trip mechanism that operates consistently under all conditions of bimetal deflection.

Another object is to provide an improved circuit breaker embodying a thermal magnetic trip mechanism that trips magnetically upon the occurrence of relatively low overload current without the use of an expensive coil or a relatively large iron-mass magnetic core member.

Another object is to provide an improved circuit breaker that can be manufactured in quantity and easily calibrated with the assurance that, between breakers, there will be no great variance in the amount of current that will be necessary for magnetic tripping. g

Another object is to provide a circuit breaker embodying an improved thermal magnetic trip mechanism having an air gap between the armature and magnetic core member, which air gap will remain constant over along period of continuous use of the circuit breaker.

Another object is to provide an improved circuit breaker embodying a thermal magnetic trip mechanism that will operate dependably and consistently without being afiected by shrinkage or war-page of the molded insulating circuit breaker housing.

Another object is to provide a circuit breaker embodying a thermal magnetic trip mechanism that is dependable, inexpensive, and easy to manufacture and assemble.

The novel features that are considered characteristic of the invention are set forth in panticular in the appended claims. The invention itself, however, both as to structure and operation, together with additional objects and advantages thereof, will be best understood from the following detailed description when read in conjunction with the accompanying drawings, in which:

FIGURE 1 is a side elevational view of a circuit breaker embodying the principles of the invention. The breaker is shown with the cover removed, and in a closed position with the open position of the movable contact and operating handle being shown in dot-and-dash lines;

PEG. 2 is a View similar to FIG. 1 except that the parts are shown in the tripped position;

HG. 3 is an exploded perspective view on an enlarged scale of some of the circuit breaker me hanism;

FIG. 4 is an end view on a smaller scale of the assembled duplex breaker;

FIG. 5 is an elevational view on an enlarged scale of the operating member showing the other side of this membar from that shownin FIGS. 1, 2 and 3;

FIG. 6 is a v'ew taken along line Vl-VI of FIG. 5;

PEG. 7 is an elevational view illustrating a different embodiment of the thermal-magnetic trip mechanism;

FIG. 8 is a side elevational view of the bimetal element shown in FIG. '7;

FIG. 9 is a side elevational view of the armature shown in HG. 7;

FIG. 10 is a top plan view of the armature as shown in FIG. 9;

HG. 11 is a top plan view of the magnetic yoke shown in FIG. 7; and I FIG. 12 is an elevational view illustrating another embodiment of the thermal-magnetic trip mechanism.

Referring to FIG. 4 of the drawings, the circuit breaker insulating housing is composed of two parts, ll and 13, forming two compartments. Each of the parts 11 and 13 is composed of a back portion molded integral with four sides forming an open front. The open front of the part ll is covered by the back portion of the part 13, and the open front of the part 13 is covered by a cover 15. The three portions ll, 13 and 15 of the housing are held rigidlytogether by three bolts 17 which extend through three openings 13 (FIGS. 1 and 2) in each of the three housing portions, and which have three nuts 19 threadedly engaging the ends thereof. Flush rivets maybe used in place of the bolts where the duplex breaker is to be mounted in close enga ement with other breakers in a row.

The housing parts ill and 13 form two independent compantments housing two circuit breaker mechanisms which, except for a line terminal structure that will be described later, are of identical construction and operation, each operating independently of the other. For this reason, only the mechanism enclosed by the housing part 13 will be specifically described, it being understood that, unless otherwise mentioned, the description applies toboth mechanisms of the duplex circuit breaker. 3

Referring to FIG. 1 of the drawing, the circuit breaker mechanism, enclosed by the housing part 13, comprises a stationary contact 21, a cooperating movable contact 23, a supporting metal frame indicated generally at 25, an operating mechanism indicated generally at 27 and a trip device indicated generally at 29.

The stationary contact 21 is Welded, or otherwise attached, to a line terminal 31 which has a flange upper portion 32 (FIG. 3) that fits into a slot in the housing pant 13. The line terminal 311 is held firmly in place by a stamped-out resilient clip portion 33 that biases against a projection 34 (shown by broken lines in FIGS. 1 and 2) which is molded integral with the housing part 13. A portion of the line terminal 31 protrudes through an opening 35 in the housing part 13 (FIG. 4). A similar portion of a symmetrically constructed line terminal 37 protrudes through an opening 39 in the housing part 11. The line terminals 31 and 37 are independent in that each is a part of a separate independently functioning circuit breaker mechanism. These terminals are each resiliently biased to engage opposite sides of a blade in a load center when the duplex breaker is mounted in operating position.

Referring to FIGS. 1, 2 and 3, the stationary contact Ell cooperates with a movable contact 23 that is welded or otherwise attached to a small flange of a flat metallic generally (s-shaped contact or switch arm 41. A bearing 42 on the operating member 4'7 has a slot 43 that is complementary with a V-shaped slot 44 in the upper portion of the movable contact arm 41. Two upper legportions 45 of the bearing 42 fit into two slots 46 in the molded insulating operating member 47. The bearing 42 transmits motion from the operating member 47 to the movable contact arm ll when the breaker is manually operated, and, as will be later explained, from the movable contact arm to the operating member when the breaker is tripped automatically in response to an overload current.

The operating member 47 has an arcuate trunnion 51 molded at each side thereof. The trunnions 51 fit and rotatably ride within two arcuate surfaces 55 on the frame 25 one of which is part of an extension 57 (FIG. 3) projecting out from the frame 25. The operating member 47 is rotatably supported between the surfaces d5 of the frame 2 5 and the bearing 42 which is supported by the provided for connection at the load terminal.

contact arm 41. The operating member has a handle portion 59 molded integral therewith which extends through an opening 61 (FIG. 2) in the housing part 13 whereby the mechanism may be manually operated to open and close the breaker. Arcuate surfaces 63 on opposite sides of the handle 59 substantially close the open ing 61 in all positions of the operating member 47.

The frame 25 supports an insulating pivot 65 (FIG. 3) having shoulders 67 at opposite ends thereof, which shoulders rest within a slot 69 in the frame 25 and a slot 71 in a projection 73 of the frame 25. A metal trip member 79 is pivotally supported at one end 77 by a bight portion 75 which is pivotally supported in a slot 31 in the insulating pivot 65. The other end 82 of the trip member 79 has a latch point 83 which rests on a ledge 85 on an armature 86 (FIGS. 1 and 2) to support the trip member in latched position. The armature S6 is part of the trip device 29 which will be described later. As best illustrated in FIG. 3, the ends 77 and 82of the trip member 79, are offset, and disposed in a plane which is parallel to a plane in which the main body portion of the trip member 79, is disposed. An overcenter spring 88 (FIGS. 1 and 2) is connected, under tension, at one end in an opening 89 in the contact arm 41, and at the other end in a slot 91 (FIG. 3) in a projection 93 extending from the trip member 79.

The movable contact arm 41 is connected by means of a flexible shunt 95 (FIGS. 1 and 2) to the free end of a bimetal 97 which is attached, at its other end, to a load terminal conductor 99. A load terminal screw 1% is The load terminal conductor 99 is welded or otherwise attached 'to a projection 1M extending out from the supporting frame, and it is given additional support by being looped over and welded to another projection 1113 extending out from the supporting frame 25. Thus, the load terminal conductor cannot move to affect the calibration of the trip device 29.

The closed electrical circuit through the breaker extends from the line terminal 31 through the stationary and movable contacts 21, 23, contact arm 41, flexible shunt connection 95, the bimetal element 97 and the load terminal conductor 99. Since the movable contact arm extends downwardly from its pivot point, the arc is established adjacent the bottom of the housing in an arc chute 111, one end of which is connected by a vent passage 113 to an opening in the end of the housing beneath the load terminal screw 160 (FIGS. 1 and 2).

The circuit breaker may be manually operated to open and close the contacts by operation of the insulating handle 59. Movement of the handle 59 clockwise from the full-line position (FIG. 1) to the position in which it is shown in dot-and-dash lines, carries the upper end of the contact arm 41 to the left of the line of action of the spring 88 whereupon the spring acts to move the contact arm 41 with a snap action to the open position shown partially in dot-and-dash lines in FIG. 1. A projection 199, molded integral with the housing part 13, acts as a limit stop for the movable contact arm during an opening operation. Movement of the operating handle 59 in a counterclockwise direction from the dot-and-dash position (FIG. 1) to the full line position moves the upper end of the contact arm 41 to the right of the line of action of the spring 88 which thereupon acts to move the contact arm to the closed position with a snap action. Movement of the handle 59 in either direction is limited by the surfaces 115 which strike the frame 25 at either side of the pivot 51.

Improved means are provided to effect automatic opening of the circuit breaker upon the occurrence of overload currents. The trip device 29 includes the bimetal 97, a Ushaped magnet 117, the armature $6 of magnetic material and a spring 119. The upper end of the bimetal 97 is welded or otherwise secured to the terminal conductor 99 which is secured to the projection 101 on the metal frame .25. The flexible conductor is welded or otherwise attached to the lower or free end of the bimetal 97, and it electrically connects the bimetal with the movable contact arm 41. The armature 86 has two generally L -shaped projections 121 that rest upon two shoulder portions 12.3 of the bimetal 97 to pivotally support the armature 86 on the bimetal 97. A tall portion 125 of the armature 36 is provided to contact the bimetal 97 to limit counterclockwise movement of the armature 86. The

U-shaped magnet yoke 117 is welded or otherwise attached to the bimetal 97 with the lower edges of its legs resting on two shoulders 127 of the bimetal. A stamped projection 129 on the magnet 117 contacts the bimetal 97 to create an air gap between the bimetal and a portion of the magnet so that the closed loop portion of the generally U-shaped spring 119 can fit between the bimetal 97 and magnet 117 in order to support the spring. The free ends of the U-shaped spring 119 engage the armature 86 to bias the armature in a counterclockwise direction. A rectangular opening in the armature 36 forms a lower edge 85 that engages the latch point 83 of the trip memher 7 9 to support the trip member in the latched position shown in FIG. 1.

Upon the occurrence of an overload current below a predetermined value, the bimetal element 97 becomes heated, and when heated a predetermined amount, defleets to the right as seen in FIG. 1. Due to the engagement of the tail portion 125 of the armature 86 with the bimetal 97, the armature is carried to the right with the bimetal to release the trip member 79. When the trip member 79 is released, the spring 8% acts to rotate it clockwise about its pivot 65 until it is arrested when a stop portion 133 strikes the projection 199 of the housing part 13. During this movement, the line of action of the spring 88 moves to the right of the pivot 44 of the contact arm 41 whereupon the spring biases the contact arm in opening direction and moves it so that the line of action of the force exerted on the operating member 47 shifts across the pivot 51, whereupon the spring 88 a'ctu ates both the contact arm 41 and the operating member arm the tripped position in which these parts are shown in FIG. 2. The movements of the trip member 79 and contact arm 41 are arrested by the projection 1119. In order to provide a visual indication that the breaker has been automatically tripped open, movement of the operating member 47 is stopped in an intermediate position (FIG. 2) when a projection 135, molded integral with the operating member 47, strikes the projection 93 which extends from the trip member '79. The parts are shown in the tripped open position in FIG. 2.

Positive separation of the contacts is assured during a tripping operation by the provision of a projection 13-7 extending from the trip member 79. If the contacts are slow in opening due to sticking, drag or other reasons, the projection 137 engages the inner edge of the contact arm 41, with a swiping action, to start the contact arm in opening direction.

Before the contacts can be closed following an automatic operation, it is necessary to reset and relatch the mechanism. This is acomplished by moving the operating handle 59 counterclockwise, from the tripped position (FIG. 2), slightly beyond the full open position in which it is shown in dotand-dash lines in FIG. 1. During this movement, due to the engagement of the projection 135 of the operating member 4 7 with the projection 93 of the trip member 79, the trip member is moved counterclockwise until the latch point 83 is again supported in the latched position on the ledge 85 of the armature 86.

The circuit breaker is tripped automatically and instantaneously by the electromagnet 97, 1-17, $6, in response to overload currents above the predetermined value. Upon the flow of current through the bimetal 97, a magnetic flux, which is induced around the bimetal, takes the path of least reluctance through the magnet 1 17, across an air gap 139, and through the armature 86! When an over-' load current above the predetermined value occurs, the pull of the magnetic flux is of such strength that the armature 86 is attracted to the magnet 17 and pivots in a clockwise direction about the bimetal 97. This movement releases the trip member 79, and the contacts are opened in the same manner previously described in connection with the thermal tripping operation.

Means are provided to assure that the predetermined amount of magnetic air gap 139 will be constant at all times except when the bimetal actively deflects after the stress within it is relieved upon the occurrence of an overload current. Every time the circuit breaker is rel-etched after a tripping operation, assuming that the fault current which caused tripping is corrected, the air gap 139 will always dependably be the predetermined amount. Referring to PEG. 1, the bimetal 97 is supported at its upper end by the projection Mi l. The unrestrained position of the lower or free end of the bimetal 97 is farther to the left than the position in which it is shown, and it is being stressed slightly to the right by engagement of a stamped projection 114 3 on the bimetal 97 with a stop portion M ll on the trip member '79. The bimetal does not deflect when heated by an overload current until this stress is relieved. One can see that under normal current conditions the relative latched positions of the trip member 79 and the bimetal 97 will always be the same. The armature as is biased counterclockwise by the spring 13.9 assuring engagement of the armature 86 with a stop portion 148 on the trip member 79. Because the relative positions of the trip member '79 and bimetal 97 are constant, and because the relative positions of the trip member 7 9 and armature 86 are constant, the air gap 139, between the armature 86 and magnet 117 which is supported by the bimetal N, must be constant. This positive positioning also assures that the amount of latch engagement between the latch point 8-3 of the trip member 7% and the latching surface 85 of the armature 86 will also be constant.

Upon magnetic tripping, the armature so, which is attracted by the magnet 117, pivots clockwise about the bimetal 97 to release the trip member 79. When the parts are in the latched position shown in FIG. 1, there is a small air gap 144 between the tail portion 125 of the armature 86 and the lower or free end of the bimetal 97. Upon thermal tripping, the bimetal 97 first relieves the stress within itself and then deflects to the right away from the trip member 79. During this movement, the armature se is biased counterclockwise about the bimetal 97 by the spring 11? closing the small air gap 144 until the tail portion 125 of the armature 86 engages the lower or free end of the bimetal 97. Further deflection of the bimetal '97 then carries the armature 86 to the right to release the trip member 79. In this embodiment of the invention, as the small air gap 144 closes upon thermal bimetal deflection, the magnetic air gap 139 is enlarged. The magnetic air gap 139, therefore, is constant at all times except when the bimetal 97 actively deflects. The purpose of the small air gap 144 is to assure that it is the stop portion 143 on the trip member 79, and not the tail portion 125 on the armature 85, which limits counterclockwise movement of the armature 86 to determine the amount of magnetic air gap 139 when the parts are in a latched position. The stop portions 148 and Ml are very positive die cuts on the trip [member 7 which will remain constant during the life of the die. By controlling the air gap 139 with the stop portions 148 and 14d, therefore, circuit breakers can be produced in quantity with the assurance that each will have the exact predetermined amount of magnetic air P- For certain applications of the circuit breaker, it is desirable that the magnetic air gap 139 be constant under all current conditions, except of course, when an overload occurs which is of sufiicient magnitude to effect a magnetic tripping operation. For these applications, the tail portion 125 of the armature 86, can be bent over so that 6 it engages the bimetal 97 at the same time that the stop portion 148 on the trip member 79 limits counterclockwise movement of the armature 86. In this modification, the armature 86 moves at all times as a unit with the bimetal 97, and the magnetic air gap 139 is constant under all conditions of bimetal deflection. The magnetic air gap 139, therefore, is constant under all current conditions except when the current is of sufficient magnitude to eifect a magnetic tripping operation.

This invention provides magnetic tripping upon the occurrence of a relatively low overload current of, for instance, ten times the circuit breaker rating, without the use of an expensive coil or a relatively large iron-mass magnetic core member. Two problems have been encountered in the past in attempts to construct a low magnetic-tripping circuit breaker that is dependable in operation. The first problem, which was solved by the present invention, was the provision of a small predetermined amount of magnetic air gap 139 that does not vary between circuit breakers and between tripping operations in the same circuit breaker. The second problem encountered was that of calibrating the trip device 29 to trip the circuit breaker with a time delay upon the occurrence of a relatively low overload current. For previous circuit breakers, it was found that the amount of bimetal deflection necessary for this thermal calibration was more than the maximum amount of magnetic air gap permissible for the low magnetic tripping and, therefore, a circuit breaker with the required small magnetic air gap could not be properly calibrated for thermal tripping. This second problem was solved by stressing the bimetal 97 in the latched position. For a clearer understanding of this second problem, let it be assumed that FIG. 1 represents, for example, a 15 ampere circuit breaker and that the bimetal 97 is not stressed in the latched positon. For the desired low magnetic tripping it is required that the air gap 139 be no more than a certain maximum, for example .02 of an inch. For proper thermal calibration, it is required that the amount of bimetal deflection necessary for a thermal tripping operation be a certain minimum, for example a dimension slightly more than .03 of an inch. If the bimetal 97 must move the armature slightly more than .03 of an inch in order to release the trip member 79, then the amount of latch engagement, between the latch point 83 on the trip member 79 and the latch surface 85 on the armature 86, must be at least .03 of an inch, and the armature 86 must move at least .03 of an inch during a magnetic tripping operation in order to release the trip member 79. It can be understood that if the magnetic air gap 139 is a maximum of .02 of an inch, the armature 86 could never move the necessary .03 of an inch for a magnetic tripping operation.

This invention solves the problem of providing for low magnetic tripping with means for thermally calibrating the trip device 29 by stressing the bimetal 97 in a latched position. The stop portions 141 and 148, on the trip member 79, are die cut to position the armature, for example, .02 of an inch away from the magnet 117. The latch point 83 is die cut, for example, a few thousandths of an inch less than .02 of an inch so that the breaker will trip magnetically. As was previously explained, the unrestrained position of the lower or free end of the bimetal 97 is farther to the left than the position in which it is shown in FIG. 1 and it is being stressed slightly to the right when the parts are in the latched position. When the bimetal 97 is heated by an overload current, this stress is first relieved before the bimetal starts to deflect in tripping direction.- The bimetal is so stressed, in a cold position, that the total amount of thermal reactionnecessary to move the bimetal .02 of an inch, in order to thermally trip the breaker, is at least equivalent to the minimum total amount of thermal reaction that would deflect the bimetal .03 of an inch if it were not stressed in a cold position. It can be understood that, in the trip device 29, there is enough bimetal thermal reaction during a thermal tripping operation to enable accurate thermal calibration,

and the magnetic air gap 139 is small enough to enable magnetic tripping upon the occurrence of relatively low overload currents.

Shrinkage or warpage of the molded insulating housing will have no effect upon the operating mechanism 27 or the trip device 29 because all moving parts are supported by the metallic frame 25. The operating member .7 is mounted between the slot 44 of the movable contact arm 41 and the surfaces 55 of the frame 25. The movable contact arm 51 is supported, at its outer end, by the bearing 42 and at its inner end by the spring 88 which is connected between the contact arm 31 and the trip member 79. The trip member 75 is pivotally supported at one end on the insulating pivot 65 which is supported in the slots 69 and 71 in the frame 25. The latch point 83 of the trip member 79 is supported by the latch surface 85 of the armature 86. The armature 86, magnetic yoke 117 and spring 119 are all supported on the bimetal 97 which is attached, at its upper end to the terminal conductor 99 which conductor is attached to the projection 191 of the frame 25. The conductor 99 is also attached to the projection 193 of the frame 25 so that movement of the outer end of the terminal conductor 99 will not be transmitted to the trip device 29.

Means are provided to compensate for reaction of the bimetal 97 in response to changes in ambient temperature. A compensating bimetal 130, having an insulating pad 132 attached to the free end thereof, is welded or otherwise attached to a projection 134 extending from the frame 25. The high expansion sides of the two bimetals are adjacent and the compensating bimetal 13th biases the bimetal 97 to the right as viewed in FIG. 1. As was previously explained, the bimetal 97 is cold stressed against the trip member 79 when the parts are in the latched position. When the bimetal 97 becomes heated upon current overload, the stress within it is first relieved and then the bimetal 97 actively deflects to the right.

'If there is a change in ambient temperature at a time when the bimetal 97 is cold stressed, the compensating bimetal 139 acts to assure that the total amount of coldstressing force exerted by the bimetal 97 against the trip member 79 remains constant, or at least varies less than it would if the compensating bimetal were not provided. If there is a rise in ambient temperature, the bimetal 97 becomes heated and the cold stress exerted by the bimetal 97 against the trip member 79 is relieved a certain amount. The compensating bimetal 130, at the same time, becomes heated and diminishes its bias against the bimetal 97 allowing the bimetal 97 to exert a greater cold-stressing force against the trip member 79. If the ambient temperature should then drop, the bimetal 97 will cool increasing its cold-stressing force against the trip member 79, and the compensating bimetal 139 will also cool increasing its bias against the bimetal 97 to allow the bimetal 97 to exert a lesser cold-stressing force against the trip member 79. It can be understood that with a proper selection of bimetals, the total amount of cold-stressing force exerted by the bimetal 97 against the trip member 79 will not be greatly varied by changes in ambient temperature.

The compensating bimetal 139 also assures that there will be no substantial change of position of the bimetal 97 in response to changes in ambient temperature at a time after the bimetal 97, heated by an overload current, has relieved the stress within itself, and is free to actively deflect. As the bimetal 97 bends to the right in response to a rise in ambient temperature, the compensating bimetal 139 bends to the left diminishing its bias against the bimetal 97. If the ambient temperature then drops, the bimetal 97 unbends towards the left, but the compensating bimetal 127 straightens out to the right increasing its bias against the bimetal 97. The net result is that there is no great change of position of the bimeta 97 and, therefore, no substantial change in the amount of latch engagement in response to changes in! the ambient temperature.

To calibrate the circuit breaker, the frame 25 and themechanism supported thereby are mounted in a nesting:

fixture, and a circuit, having a predetermined amount of current, is established from a stationary contact member that is part of the nesting fixture, through the circuit breaker mechanism. A tapered pin is then driven through an opening 136 (FIG. 3) in the frame 25 enlarging the opening 136 by narrowing a slot 138 in the metal frame 25, and stretching the metal adjacent the slot. As the opening 136 is enlarged, the bimetal 97 and armature 86 are moved to the right as viewed in FIG. 1 until the breaker trips. The tapered pin is then driven through the opening 136 an additional distance to compensate for spring-back of the frame 25. The frame and calibrated mechanism are then removed from the nesting fixture and assembled in the circuit breaker housing. An insulating projection 1% (FIGS. 1 and 2) which is molded integral with the housing part 13, passes through the frame opening 136 to strengthen the frame 25 and position it within the housing. One of the mounting openings 18 extends through the projection 14%. Another insulating projection 142 (shown in broken lines in FIGS. 1 and 2), which is molded integral with the housing part 13, passes through an opening 146 (FIG. 3) in the frame 25, and serves also to fixedly position the frame within the housing. The insulating projection 142 also serves to position the metallic trip member away from the metallic frame 25.

According to the embodiment of the invention illustrated in FIG. 7 of the drawings, the armature 86' is biased in a counterclockwise direction about the bimetal 97 by a U-shaped spring 145 that fits between the armature 86' and the bimetal 97'. The constant relative latched positions of the trip member 79' and the bimetal 97' are determined by the projection 147 of the magnet yoke 117' mounted on the bimetal 97', which projection engages a stop portion 149 on the trip member 79' when the parts are in a latched position. In all other respects, the structure and operation of the trip device 29 are the same as the trip device 29 shown in FIGS. 1, 2 and 3. Thus, the left-most position to which the bimetal can move is determined by the part 147 engaging the stop portion 149 on the trip member, and the bimetal is thereby stressed in its cold and/ or its slightly heated condition when it is carrying normal rated current.

The tail portion of the armature 86' can be bent over so that it engages the bimetal 97 at the same time that the stop portion 148' on the trip member 79' limits counterclockwise movement of the armature 86. This modification, as in the modification of the first embodiment of the invention, eliminates the small air gap 144 and assures that the armature 96' will move at all times as a unit with the bimetal 97. In this modification, the magnetic air gap 139' is constant under all current conditions except when the current is of sufficient magnitude to effect a magnetic tripping operation.

Another embodiment of the invention is illustrated in FIG. 12. This trip device 29" is the same as that shown in FIG. 1 except that the trip member 79" does not have a stop portion, equivalent to the stop portion 148 of the trip device 29, to limit counterclockwise movement of the armature 86", and there is no air gap between the tail portion 125 of the armature 86" and the free end of the bimetal 97". In this embodiment, the armature tail portion 125" engages the free end of the bimetal 97" to limit counterclockwise movement of the armature 86". In all other respects the trip device 29" is the same as the trip device 29 shown in FIGS. 1, 2 and 3. The leftmost position to which the bimetal 97" can move is determined by the projection 143" on the bimetal 97 engaging the stop portion 141 on the trip member 79", and the bimetal 97 is thereby stressed in its cold and/ or its slightly heated position when it is carrying normal rated current.

9 The air gap 139" is constant under all conditions of bimetal deflection, and therefore under all cur-rent conditions except when the current is of sufficient magnitude to effect a magnetic tripping operation.

The embodiment illustrated in FIG. 12 does not have the advantages, of the first two embodiments, of having the amount of small magnetic air gap controlled by two stop portions on a trip member which stop portions are positive die cuts assuring that circuit breakers produced in quantity will each have the exact predetermined amount of magnetic air gaps. The advantage of this embodiment is that it provides a trip device with a small predetermined amount of magnetic air gap that is constant under all conditions of bimetal deflection which air gap is determined by only two sets of engaging parts; namely the engagement of the stop portion 141" on the trip member 79" with the projection 143" of the bimetal 97", and the engagement of the tail portion 125" of the armature 86 with the lower part of the bimetal 97".

(Iertain features of this circuit breaker are described and claimed in the copending application of Francis L. Gelzheiser, Serial No. 850,650, filed November 3, 1959, and assigned to the assignee of the instant application.

While the invention has been disclosed in accordance with the provisions of the patent statutes, it is to be un erstood that various changes in the structural details and arrangement of parts thereof may be made without departing from some of the essential features of the invention. it is desired, therefore, that the language of the appended claims be given as reasonably broad an interpretation as is permitted by the prior art.

I claim as my invention:

1. In a circuit interrupter having separable contacts and latched means releasable to effect separation of said contacts, a bimetallic member heated in response to current flow, a magnet, an armature movable by movement of said bimetallic member to effect release of said latched releasable means, a stop against which said bimetallic member is stressed in its cold condition, said bimetallic member remaining against said stop during the initial part of its heating while said stress is relieved, said bimetallic member then moving after said stress is relieved to move said armature in response to overload currents below a predetermined value to effect release of said latched releasable means, and said armature being attracted to said magnet and moving relative to said bimetallic member in response to overload currents above said predetermined value to effect release of said latched releasable means.

2. A circuit interrupter including separable contact means, releasable means for causing interruption of the circuit at said contact means, an armature movable to a tripping position to effect release of said releasable means, a magnet having a small air gap from said armature suii'lcient to permit movement of the armature magnetically to said tripping position before the air gap is closed, a bimetal connected to move said armature and heated by current flow, a stop against which said bimetal is stressed when traversed by normal rated current, said bimetal when heated by low overload currents first relieving the stress against said stop without effective movement of said armature and then moving away from the stop and moving the armature to its tripping position to efiect release of said releasable means.

3. A circuit interrupter including separable contact means, releasable means for causing interruption of the circuit at said contact means, an armature movable to a tripping position to effect release of said releasable means, a magnet having a small air gap from said armature sufficient to permit movement of the armature magnetically to said tripping position before the air gap is closed, a bimetal connected to move said armature and heated by current flow, a stop against which said bimetal is stressed when traversed by normal rated current, said bimetal when heated by low overload currents first relieving the stress against said stop without effective movement of said armature and then moving away from the stop and moving the armature to its tripping position to effect release of said releasable means, a compensating bimetal normally applying a force to the first said bimetal in the direction to reduce the stress of the first said bimetal on said stop, and said compensating bimetal exerting less stress on the first said bimetal upon an increase in ambient temperature while the first said bimetal remains stressed against said stop with a stress which must be thermally relieved before the first said bimetal begins to move away from said stop.

4. A circuit interrupter including separable contact means, releasable means for causing interruption of the circuit at said contact means, an armature movable to a tripping position to effect release of said releasable means, a magnet having a small air gap from said armature sulficient to permit movement of the armature magnetically to said tripping position before the air gap is closed, a bimetal connected to move said armature and heated by current flow, a stop on said releasable means against which said bimetal is stressed when traversed by normal rated current, a second stop on said releasable means against which said armature normally rests, whereby the normal relative position of said bimetal and armature is determined by the relative positions of said two stops on said releasable means.

5. A circuit interrupter including separable contact means, releasable means for causing interruption of the circuit at said contact means, an armature movable to a tripping position to effect release of said releasable means, a magnet having a small air gap from said armature sufiicient to permit movement of the armature magnetically to said tripping position before the air gap is closed, a bimetal connected to move said armature and heated by current flow, said bimetal having said magnet mounted thereon to be moved by the bimetal, a first stop against which said bimetal is normally stressed, a second step limiting the movement of said armature away from said magnet, and said two stops having a fixed dimension therebetween whereby a constant air gap is maintained between the magnet and armature while the stress of the bimetal on said first stop is relieved during initial heating of the bimetal.

6. In an automatic circuit breaker having a stationary contact and a movable contact cooperating therewith to open ad close the circuit, means releasable to effect automatic opening of said contacts, a bimetal element, a member of magnetic material supported by said bimetal element, an armature supported by said bimetal element, said bimetal element upon being heated by an overload current below a predetermined value deflecting and moving to effect release of said releasable means, said member of magnetic material upon being energized by an overload current above said predetermined value attracting said armature causing said armature to move to effect release of said releasable means, and means stressing said bimetal element to increase the amount of thermal reaction necessary llO deflect said bimetal element to effect release of said releasable means.

7. A circuit breaker comprising, in combination, a stationary contact and a movable contact cooperating with said stationary contact to open and close a circuit, a trip member releasable to eifect automatic opening of said contacts, a bimetal element stressed in its cold position, a magnetic core member supported by said bimetal element, a first means positively positioning said stressed bimetal element relative to said ltrip member, an armature supported by said bimetal element and movable to release said trip member, a second means positively positioning said armature relative to said trip member when said bimetal element is stressed, said bimetal element bending and moving said armature in response to overload currents below a predetermined value to release said releasable means, and said armature upon the 11 p v occurrence of overload currents above said predetermined value moving towards said magnetic core member to release said latched releasable means.

'8. In an automatic electric circuit breaker having separable contacts and latched means releasable to effect separation of said contacts, a bimetal member having a shoulder portion thereon, a magnet member supported on said bimetallic member, an armature having a projection intermediate the ends thereof resting on said shouider portion to pivotally support said armature on said bimetallic member, a latching portion on said armature latching :sa'id latched releasable means, said bimetallic member bending and carrying with it said armature in response to an overload current below a predetermined value to release said latched releasable means, and said magnet member upon being energized sufliciently by an overload current above said predetermined value attracting said armature causing said armature to pivot relative to said bimetallic member to release said latched releasable means.

9. In a circuit interrupter having separable contacts and latched means releasable to effect separation of said contacts, a bimetallic member having a shoulder portion at each side thereof, a magnetic core member supported upon said bimetallic member, an armature having anupper and lower portion and a projection at each side thereof intermediate said portions, said projections resting on said shoulder portions to pivotal-1y support said armature on said bimetallic member, means biasing said upper portion away from said bimetallic member, a latching surface on said upper portion latching said latched releasable means, said bimetallic member in response to overload currents below a predetermined value bending and engaging said lower portion of said armature to move said armature with it as a unit to release said latched releasable means, and said magnetic core ember upon being energized sufficiently by an overload current above said predetermined value attracting said upper portion of said armature causing said armature to pivot relative to said bimetallic member to release said latched releasable means.

10. A circuit breaker including, in combination, a stationary contact, a movable contact cooperating with said stationary contact to open and close said circuit breaker, a latched trip member releasable to effect automatic opening of said contact, a bimetal element stressed in position when normal current or less than normal current is flowing through said circuit breaker, means on said trip member engaging means on said bimetal element stressing said bimetal element to assure positive positioning of said bimetal element relative to said trip member, an armature movably supported by said bimetal element and latching said latched trip member, a magnetic member supported by said bimetal element, means on said armature engaging means on said trip member to positively position said armature relative to said magnetic member when said bimetal element is in a stressed position, said bimetal element in response to overload currents below a predetermined value bending and moving said armature to release said latched trip member, and said magnetic member in response to overload currents above said predetermined value attracting said armature to release said latched trip member.

11. A circuit breaker includin in combination, a stationary contact, a movable contact cooperating with said stationary contact to open and close said circuit breaker, a latched trip member releasable to effect automatic opening of said contacts, said trip member having a first stop portion and a second stop portion, a bimetal element having a projection thereon engaging said first stop portion to position said bimetal element relative to said trip member, a magnetic core member attached to said bimetal element, an armature movably supported on said bimetal element and having an opening therein denning a ledge, said trip member resting on said ledge 12 in a latched position, said mcond stop portion engaging said armature and positioning said armature relative to said magnetic core member, said armature being movable to release said latched trip member, said bimetal element bending and moving said armature in response to overload currents below a predetermined value to release said releasable means, and said magnetic core mem ber upon being energized sufficiently by overload currents above said predetermined value attracting said armature causing said armature to move relative to said bimetal element to release said latched trip member.

12. A circuit breaker including, in combination, a stationary contact, a movable contact cooperating with said stationary contact to open and close said circuit breaker, a latched trip member releasable to effect automatic opening of said contacts, said trip member having a first stop portion and a second stop portion, a bimetal element, a magnetic core member attached to said bimetal element and having a projection thereon engaging said first stop portion to position said bimetal element relative to said trip member, an armature movably supported by said bimetal element and having an opening therein defining a ledge, said trip member resting on said ledge in a latched position, said second stop portion engaging said armature and positioning said armature relative to said magnetic core such that an air gap between said armature and said magnetic yoke remains constant during normal operation of said circuit breaker, said armature being movable to release said latched trip member, said bimetal element bending and moving said armature in response to overload currents below a predetermined value to release said releasable means, and said magnetic core member upon being energized sufficiently by overload current-s above said predetermined value attracting said armature causing said armature to move relative to said bimetal element to close said air gap and release said latched trip member.

13. A circuit breaker including, in combination, a stationary contact, a movable contact cooperating with said stationary contact to open and close said circuit breaker, latched means releasable to effect opening of said contacts, a bimetal element, a magnetic member supported on said bimetal element, an armature pivotally supported intermediate the ends thereof upon said bimetal element, one end of said armature resting against said bimetal element, means on said armature latching said latched releasable means, said bimetal element bending and moving said armature with it as a unit in response to overload currents below a predetermined value to release said latched releasable means, and said armature being attracted to said magnetic member and pivoting on said bimetal element in response to overload currents above said predetermined value to release said latched releasable means.

14. In a circuit interrupter having separable contacts and latched means releasable to effect automatic separation of said contacts, a bimetallic member having a shoulder portion, a magnetic member supported upon said hi metallic member, an armature having a first and second end, a projection intermediate said ends resting on said shoulder portion to pivotally support said armature on said bimetallic member, means biasing said first end a distance away from said bimetallic member, said disstance being determined when said second end engages said bimetallic member, a latching portion on said armature on the first end side of said projection, said latching portion latching said latched releasable means, said bimetallic member bending and moving said armature in response to an overload current below a predetermined value to release said latched releasable means, and said magnetic member being energized sufiiciently by an overload current above said predetermined value attracting said first-end side of said armature causing said armature to pivot relative to said bimetallic member to release said latched releasable means.

15. A circuit breaker including, in combination, a stationary contact, a movable contact cooperating with said stationary contact to open and close said circuit breaker, latched means releasable to efiect automatic opening of said contacts, a bimetallic member having a shoulder portion at each side intermediate the ends thereof, a magnetic core member supported by said bimetallic member, an armature having a first and second end, a projection on each side of said armature intermediate said first and second ends, said projections resting on said shoulder portions to pivotally support said armature on said himetallic member with the first-end part of said armature opposite said magnetic core member, means biasing said first-end part away from said magnetic core member creating an air gap between said first-end part and said magnetic core member, said second-end part of said armature engaging said bimetallic member, said biasing means and said second-end part assuring that said air gap will be constant when the current flowing through said circuit breaker is less than a predetermined value, a latching surface on the first-end part of said armature, said latching surface engaging said latched releasable means to support said latched releasable means in la latched position, said bimetallic member bending and moving said armature in response to an overload current below said predetermined value to release said latched releasable means, and said magnetic core member when energized by an overload current above said predetermined value attracting said first-end part of said armature causing said armature to pivot relative to said bimetallic member to release said latched releasable means.

16. A circuit breaker including a metallic frame and a circuit breaker mechanism; said circuit breaker mechanism including a stationary contact, a movable contact, a trip member and a trip device; said movable contact cooperating with said stationary contact to open and close said circuit breaker; said trip member being releasable to effect automatic opening of said contacts; said trip device including a bimetal element, a magnetic member supported on said bimetal element, and an armature pivotally supported intermediate its ends on said bimetal element and movable to release said trip member; said bimetal element moving in response to overload currents below a predetermined value and moving said armature with it as a unit, said movement of said armature effecting release of said trip member; said armature in response to overload currents above said predetermined value being attraced to said magnetic member to release said trip member; and said circuit breaker mechanism being supported on said metallic frame.

17. A circuit breaker including, in combination, a stationary contact and a movable contact member cooperating therewith to open and close said circuit breaker, a trip member releasable to move and effect automatic opening of said contacts, a bimetallic member, a magnetic core member supported by said bimetallic member, an armature supported by said bimetallic member and movable to release said trip member, said bimetallic member in response to overload currents below a predetermined amount moving and carrying said armature with it as a unit to release said trip member, said armature upon the occurrence of overload currents above said predetermined value being attracted to said magnetic core member and moving to close an air gap between said armature and said magnetic core member thereby releasing said trip member, stop means on said trip member engaging said bimetallic member and said armature and serving to maintain said air gap constant under normal current conditions, a metallic frame, and all of said movable structure being supported by said metallic frame.

18. In a circuit breaker having separable contacts and a latched trip member releasable to effect automatic sepa ration of said contacts, a latching bimetal element, a magnetic member supported by said latching bimetal element,

an armature movably supported on said latching bimetal element, means on said armature engaging said trip member to hold said trip member in a latched position, said armature being movtble to release said trip member, said bimetal element in response to overload currents below a predetermined value bending and moving said armature with it as a unit to release said trip member, said armature in response to overload currents above said predetermined value being attracted to said magnetic member and moving relative to said latching bimetal element to release said trip member, a generally straight compensating bimetal element exerting a bias against said latching bimetal element, the high expansion sides of said latching bimetal element and said compensating bimetal element being adjacent, and said compensating bimetal element providing that the latching position of said armature is not substantially varied by changes in ambient temperature.

19. A circuit breaker including, in combination, a stationary contact, a movable contact cooperating with said stationary contact to open and close said circuit breaker, a latched trip member releasable to effect automatic opening of said contacts, a latching bimetallic member fixed to a stationary support, a magnetic core member supported by said latching bimetallic member, an armature supported by said latching bimetallic member and having means thereon engaging said trip member to hold said trip member in a latched position, said armature being movable to release said trip member, said bimetallic member in response to overload currents below a predetermined value moving and carrying said armature with it as a unit to release said trip member, said magnetic core member energized by overload currents above said predetermined value attracting said armature to release said trip member, means for maintaining the relative positions of said armature and said magnetic core member constant When a normal amount of current is flowing through said circuit breaker, a generally straight compensating bimetallic member, said compensating bimetallic member being fixed to a stationary support and being disposed adjacent said latching bimetallic member, said compensating bimetallic member normally exerting a bias against said latching bimetallic member, the high expansion sides of said latching bimetallic member and said compensating bimetallic member being adjacent, and said compensating bimetallic member compensating for movement of said latching bimetallic member in response to changes in ambient temperature to provide that the latching position of said armature is not substantially varied by changes in ambient temperature.

20. A circuit breaker including, in combination, a stationary contact and a movable contact member cooperating therewith to open and close said circuit breaker, a trip member releasable to move and eiiect automatic opening of said contacts, a tripping bimetal member, a magnetic core member supported by said bimetal member, an armature supported by said tripping bimetal member and movable to release said trip member, said tripping bimetal member heated by overload currents below a predetermined amount moving and carrying said armature with it to release said trip member, said magnetic core member energized by overload currents above said predetermined value attractin; said armature to release said trip member, a compensating bimetal member exerting a bias against said latching bimetal member under stationary contact and a movable contact cooperating with said stationary contact to open and close a circuit, a latched trip member having a first stop means and a second stop means thereon and being releasable to effect automatic opening of said contacts, a bimetal member, a magnetic member fixedly supported on said bimetal member, said first stop means positively positioning said bimetal member relative to said trip member, an armature movably supported on said bimetal member and movable to effect release of said trip member, said second stop means positively positioning said armature relative to said magnetic member, said bimetal member bending in response to overload currents below a predetermined value to effect release of said releasable means, and upon the occurrence of overload currents above said predetermined value said armature moving toward said magnetic member to effect release of said releasable means.

22. A circuit breaker comprising, in combination, a stationary contact, a movable contact cooperating with said stationary contact to open and close said circuit breaker, a latched trip member releasable to eifect opening of said contacts, said trip member having a first stop portion and a second stop portion thereon, a current-carrying bimetal member, a magnetic member fixedly supported on said bimetal member, an armature movably supported on said bimetal member, when the current flowing through said bimetal member is below a first predetermined value said first stop portion serving to positively position said bimetal member relative to said trip member and said second stop portion serving to positively position said armature relative to said bimetal member whereby an air gap between said armature and said magnetic member remains constant, upon the occurrence of an overload current higher than said first predetermined value and lower than a second predetermined value said bimetal member flexing and moving said armature therewith to effect release of said trip member, and upon the occurrence of an overload current above said second predetermined value said armature being attracted to said magnetic member and moving relative to said magnetic member and bimetal member to efifect release of said trip member.

23. A circuit breaker comprising, in combination, a stationary contact, a movable contact cooperating with said stationary contact to open and close said circuit breaker, a latched trip member releasable to effect automatic opening of said contacts, said trip member having a first stop portion and a second stop portion thereon, a bimetal element having a projection thereon engaging said first stop portion to position said bimetal element relative to said trip member, a magnetic core member attached to said bimetal element, an armature movably supported on said bimetal element and being movable to eifect release of said latched trip member, said second stop portion engaging said armature and positioning said armature relative to said magnetic core member, said bimetal element bending and moving said armature in response to overload currents below a predetermined value to effect release of said releasable means, and said magnetic core member upon being energized sufiiciently by overload currents above said predetermined value attracting said armature causing said armature to move relative to said bimetal element to effect release of said latched trip member.

24. A circuit breaker comprising, in combination, a stationary contact, a movable contact cooperating with said stationary contact to open and close said circuit breaker, a latched trip member releasable to effect automatic opening of said contacts, said trip member having a first stop portion and a second stop portion thereon, a bimetal element, a magnetic core member attached to said bimetal element and having a projection thereon engaging said first stop portion to position said bimetal element relative to said trip member, an armature movably sup- IFWiCd 9 sa d bimetal element and being movable to eifect release of said latched trip member, said second stop portion. engaging said armature to position said armature relative to said magnetic core member, said bimetal element bending and moving said armature in response to overload currents below a predetermined value to eifect release of said releasable means, and said magnetic core member upon being energized sufficiently by overload currents above said predetermined value attracting said armature causing said armature to move relative to said bimetal element to effect release of said latched trip member.

25. A circuit breaker comprising separable contact means, a releasable trip member releasable to eifect interruption of the circuit at said contact means, an armature movable to a tripping position to effect rel-ease of said releasable means, a magnet disposed such that there is an air gap between said armature and said magnet sufficient to permit movement of the armature magnetically to said tripping position before the air gap is closed, a bimetal connected to move said armature and heated by current flow, said magnet being rigidly mounted on said bimetal, said trip member having a first stop portion and a second stop portion thereon, said first stop portion serving to effect a stressing of said bimetal under normal current conditions, means biasing said armature away from said magnet, said second stop portion serving to limit the movement of said armature away from. said magnet under normal current conditions, and said two stop portions being fixed on said trip member whereby a constant air gap is maintained between said magnet and said armature under normal current conditions.

26. A circuit breaker comprising, in combination, a stationary contact, a movable contact arm having a contact thereon, an operating handle, overcenter spring means operable upon movement of said operating handle to move said contact arm to open and close said contacts, a releasable trip member separate from said contact arm, upon release of said trip member said overcenter spring means being operated to move said contact arm to open said contacts, a bimetal member separate from said cont-act arm, said bimetal member being heated upon the occurrence of an overload current below a predetermined value and moving to effect a thermal release of said trip member, magnetic means comprising a magnetic member and an armature both of which are supported on said bimetal member, upon the occurrence of an overload current above said predetermined value said magnetic means operating to efiect a magnetic release of said trip member, and means stressing said bimetal member under normal ambient and current conditions to thereby increase the amount of thermal reaction of said bimetal member necessary to move said bimetal member to eifect a thermal release of said trip member.

27. A circuit breaker comprising, in combination, a stationary contact, a movable contact arm having a contact thereon, an operating handle, overcenter spring means operable upon movement of said operating handle to move said contact arm to open and close said contacts, a releasable trip member separate from said contact arm, upon release of said trip member said overcenter spring means being operated to move said contact arm to open said contacts, a bimetal member separate from said contact arm, said bimetal member being heated upon the occurrence of an overload current below the predetermined value and moving to effect a thermal release of said trip member, magnetic means comprising a magnetic member and an armature, at least one of said magnetic means be ing supported on said bimetal member, upon the occurrence of an overload current above said predetermined value said magnetic means operating to effect a magnetic release of said trip member, and means stressing said bimetal member under normal ambient and current conditions to increase the amount of thermal reaction of said bimetal member necessary to move said bimetal member to effect a thermal release of said trip member.

28. A circuit breaker having a stationary contact and a movable contact member cooperable therewith to open and close said circuit breaker, a latched trip member releasable to open said circuit breaker, a bimetal latching member latching said trip member, said bimetal latching member bending in response to abnormal current conditions to release said trip member, a bimetal compensating member biasing said bimetal latching member to a flexed position, the high expansion sides of said bimetal latching member and said bimetal compensating member being adjacent, and said bimetal compensating member compensating for reaction of said bimetal latching member in re sponse to changes in ambient temperature.

29. A circuit breaker having a stationary contact and a movable contact member cooperating therewith to open and close said circuit breaker, a latched trip member releasable to open said circuit breaker, a bimetal latching member latching said trip member, said bimetal latchingmember bending away from said trip member in response to certain overload currents below a predetermined value to thereby release said latched trip member, electromagnetic means for bending said bimetal latching mem her away from said trip member in response to certain overload currents above said predetermined value to effect instantaneous release of said latched trip member, a bimetal compensating member biasing said bimetal latching member to a flexed position in tripping direction, the high expansion sides of said bimetal latching member and said bimetal compensating member being adjacent, said bimetal compensating member diminishing and increasing the bias on said bimetal latching member in response to changes in ambient temperature.

References Cited in the file of this patent UNITED STATES PATENTS Von Hoorn Nov. 3, Randall May 19, Randall Nov. 30, Sachs Sept. 27, Sachs July 30, Fisher Aug. 27, Von Hoorn lune 17, Jackson Sept. 2, Jackson et a1 Jan. 17, Jackson et a1 Jan. 2, Walker Sept. 18, Casey Oct. 30, Jackson Dec. 25, Swingle Nov. 25, Bingenheimer Ian. 6, Bingenheimer Apr. 27, Jackson et a1 Dec. 14, Aulbert Mar. 1, Bingenheimer et al Nov. 12, Stanback et al Sept. 1, Middendorf Apr. 26,

FOREIGN PATENTS Great Britain I an. 15, Australia June 30, 

1. IN A CIRCUIT INTERRUPTER HAVING SEPARABLE CONTACTS AND LATCHED MEANS RELEASABLE TO EFFECT SEPARATION OF SAID CONTACTS, A BIMETALLIC MEMBER HEATED IN RESPONSE TO CURRENT FLOW, A MAGNET, AN ARMATURE MOVABLE BY MOVEMENT OF SAID BIMETALLIC MEMBER TO EFFECT RELEASE OF SAID LATCHED RELEASABLE MEANS, A STOP AGAINST WHICH SAID BIMETALLIC MEMBER IS STRESSED IN ITS COLD CONDITION, SAID BIMETALLIC MEMBER REMAINING AGAINST SAID STOP DURING THE INITIAL PART OF ITS HEATING WHILE SAID STRESS IS RELIEVE, SAID BIMETALLIC MEMBER THEN MOVING AFTER SAID STRESS IS RELIEVED TO MOVE SAID ARMATURE IN RESPONSE TO OVERLOAD CURRENTS BELOW A PREDETERMINED VALUE TO EFFECT RELEASE OF SAID 